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BIG PHYSICS, BIG QUESTIONS –

Runaway star may have spawned the solar system

By Rachel Courtland

The solar system may have been born inside the remains of a single star that ran away from its family, rather than from a tight-knit clan of stars. If so, it may be more unusual than previously thought.

Meteorites that contain bits of rock called calcium-aluminium-rich inclusions suggest that the solar system may have formed very quickly from the ashes of other stars. That’s because the inclusions formed with the radioactive isotope aluminium-26, which is forged inside stars tens of times as massive as the sun and decays with a half-life of only 720,000 years.

Such massive stars tend to form in clusters, and they shed material in roiling winds that can cool down and seed planetary systems.

Too hot

But Vincent Tatischeff of the National Center for Scientific Research in Orsay, France, and colleagues suspect a massive star cluster would have been have been so hot that most of the Al-26 would have decayed before planets could congeal.

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Instead, they suggest the solar system sprang from a solitary star’s ashes, which could have cooled more quickly. To account for the amount of Al-26 observed in meteorites, the star would still have had to be massive, meaning it probably formed in a clutch of other stars.

At some point, it may have been flung out of its birth cluster by gravitational tussles with its siblings or the explosion of a companion. “The scenario may look complicated, but we think it is the most likely origin of the aluminium-26 in the solar system,” Tatischeff says.

Ocean worlds

As it zipped through interstellar space, the star would have released Al-26 in winds, forming a shell of material around it. When the star later exploded, its remains would have slammed into this shell, creating a turbulent region with areas dense enough for the sun to form.

Tatischeff says most of the galaxy’s planetary systems may not have formed as quickly as ours, since many probably arose from clusters. This makes them likely to have lower levels of Al-26, which generates heat as it decays. The cooler temperatures may have led rocky planets to take a different evolutionary path to Earth, perhaps becoming ocean worlds.

Eric Gaidos of the University of Hawaii at Manoa did preliminary work on the possibility of a runaway star parent last year, but says the single-star scenario still has trouble explaining how hot gas from the star could mix with surrounding material efficiently enough to form the solar system quickly. “We have yet to deal with the really tough nuts that have to be cracked,” Gaidos says.